Gallium nitride (GaN) is useful as a substance applicable to electronic devices such as light-emitting diodes, laser diodes, etc. For producing gallium nitride crystals, at present, a vapor-phase epitaxial growth method on a substrate such as sapphire, silicon carbide or the like according to an MOCVD (metal-organic chemical vapor deposition) method is the most popular. However, according to the method, GaN crystals are heteroepitaxially grown on a substrate that differ from GaN in the lattice constant and the thermal expansion coefficient, and therefore, the method is problematic in that the GaN crystals to be formed therein may often involve dislocation and lattice defect and the crystals could hardly have the quality applicable to blue laser, etc.
Recently, therefore, it has become strongly desired to establish a novel technique, which is substitutable for the above-mentioned method, for producing high-quality bulk single crystals of gallium nitride for homoepitaxial substrates. As one of such novel production methods for gallium nitride crystals, a solution growth method for a nitride using ammonia as a solvent (so-called ammonothermal method) has been proposed. R. Dwilinski, et al. have obtained gallium nitride crystals using KNH2 as a mineralizing agent for crystallization in a solvent of supercritical ammonia under a high pressure of from 100 to 500 MPa (see Non-Patent Reference 1). Kolis, et al. have obtained gallium nitride crystals using KNH2 and KI as a mineralizing agent for crystallization in a solvent of supercritical ammonia under a high pressure of 240 MPa (see Non-Patent Reference 2). Chen, et al. have obtained gallium nitride crystals using NH4Cl as a mineralizing agent for crystallization in a solvent of supercritical ammonia under a high pressure of about 200 MPa in a Pt-lined reactor (see Non-Patent Reference 3).
In these production methods for gallium nitride crystals, a pressure reactor (for example, autoclave or the like) is first cooled and then charged with ammonia after cooled. As the method for charging a reactor with ammonia, for example, there is known a method of cooling a reactor with an external coolant such as liquid nitrogen, methanol with dry ice or the like and charging the reactor with gaseous ammonia being condensed (for example, see Patent References 1 and 2). However, these cooling methods are problematic in that the charging accuracy is low. In addition, when they are applied to a large-scale reactor, the whole pressure vessel having a large calorific capacity must be cooled from its outside, and therefore the methods are disadvantageous in point of the cost for cooling equipment, mobile equipment, etc.
Also known is a method of charging a vessel directly with liquefied ammonia (for example, see Non-Patent Reference 4). According to the method, the vessel can be cooled by the latent heat of vaporization of liquefied ammonia being charged therein. Accordingly, it is unnecessary to previously cool the vessel, and the vessel may be charged with ammonia at room temperature. However, in direct charging with ammonia that is liquid, the purity of ammonia may lower owing to the impurities existing in ammonia.
Also known is a method of charging with ammonia by a plunger pump. However, the method requires high-pressure equipment and could hardly enhance the charging accuracy. Further, there is a high possibility of contamination of ammonia with impurities from pumps and pipelines, and it is difficult to increase the purity of the ammonia charged in vessels.
Patent Reference 1: JP-A 2005-289797
Patent Reference 2: JP-A 9-273837
Non-Patent Reference 1: R. Dwilinski, et al., ACTA PHYSICA POLONICA A, Vol. 88 (1995), p. 833
Non-Patent Reference 2: Kolis, et al., J. Crystal Growth 222 (2001), p. 431
Non-Patent Reference 3: Chen, et al., J. Crystal Growth 209 (2000), p. 208
Non-Patent Reference 4: Taro Shimomitsu, Liquefied Ammonia Organic Chemistry, Giho-do (1957)